Recently, interests in electric vehicles or hybrid vehicles running on electrical energy, not on fossil energy, are increasing due to concern about exhaustion of fossil energy and environmental pollution.
The electric vehicles or hybrid vehicles mainly use secondary batteries capable of charging and discharging repeatedly. Here, discharging is the conversion of chemical energy into electrical energy and charging is the conversion of electrical energy into chemical energy. The secondary batteries include nickel cadmium batteries, nickel hydrogen batteries, lithium ion batteries, lithium ion polymer batteries and so on.
In particular, a high capacity battery pack, in which a plurality battery cells are connected in series, is used in operating a high-power electric motor required to run the electric vehicles or hybrid vehicles.
It is important for the electric vehicles or hybrid vehicles to display the residual electrical energy of a battery so that a user can estimate the distance to empty. Conventional gasoline vehicles operate the engine using fuel, and thus, do not have great difficulty in measuring the amount of fuel. However, while the electric vehicles or hybrid vehicles are being charged or discharged, there is a difference between the measured properties (voltage, internal resistance and so on) and actual properties due to nonlinearity of electrochemical properties, aging effects, IR drop phenomenon and so on. Thus, the electric vehicles or hybrid vehicles have difficulty in accurately measuring electrical energy remaining in a battery during traveling. Currently, the residual electrical energy of a battery is measured by estimating the state of charge (SOC) of the battery. Meanwhile, the state of health (SOH) is also used as an indicator of the state of a battery. Because the characteristics of batteries change over time, SOH serves as an index of aging effects. The quantitative estimation of SOH allows for a user to know in advance how much electrical energy of a battery is available. A typical parameter used for SOH estimation is an internal resistance of a battery. The internal resistance is difficult to accurately measure during the use of a battery. Thus, the internal resistance is measured in the state that the use of a battery is stopped, or indirectly estimated during the use of a battery.
The internal resistance of a battery may be estimated by calculating a rate of voltage of each battery cell in the battery to a charging or discharging current of the battery. For more accurate estimation of the internal resistance, it needs to synchronize a current measurement time point with a voltage measurement time point. Here, synchronization of current and voltage measurement time points does not literally mean an exact synchronization of a current measurement time point with a voltage measurement time point, but making a time difference between a voltage measurement time point and a current measurement time point. As batteries operate by electrochemical reaction, when changes in electric current occur, it takes a predetermined time till voltages at opposite ends of the battery change, due to the internal resistance. Conventionally, since it is difficult to accurately measure current and voltage of a battery after synchronization of current and voltage measurement time points, the internal resistance is calculated by continuously measuring current of the battery at a shorter current measuring cycle than a voltage measuring cycle, measuring voltage of the battery when estimation of the internal resistance is necessary, and selecting a value of current measured immediately before the voltage measurement time point.
However, the conventional technique has a drawback that frequent sampling of electric current measurement results in overload applied on a host processor of a battery management system which controls the overall current and voltage measurement. The overload causes malfunction or operating speed reduction of the host processor of the battery management system. And, if current and voltage are measured in the state that a current measurement time point is not exactly synchronized with a voltage measurement time point, an internal resistance estimated from the measured current and voltage is not correct, either.
Meanwhile, even though a current measurement time point is synchronized with a voltage measurement time point, the synchronized state is not continuously maintained. This is because if the host processor of the battery management system suffers from performance changes due to overheat, overload and so on, a small error occurs to the synchronized state of current and voltage measuring cycles, and the error accumulates and becomes larger as time passes.